The most important development in electrolysis electrodes in recent years has been the advent of dimensionally stable electrodes following the teachings of U.S. Pat. Nos. 3,771,385 and 3,632,498. These dimensionally stable electrodes consist of a base or substrate of a valve metal, typically titanium, carrying an electrocatalytic coating such as a mixed oxide of platinum group metal and a valve metal forming a mixed crystal or solid solution. Many different coating formulations have been proposed.
The major use of these dimensionally stable electrodes has been as anodes in chlor-alkali production in mercury cells, diaphragm cells and more recently in membrane cells. Other uses have been as oxygen-evolving anodes for metal electrowinning processes, for hypochlorite and chlorate production, as metal plating anodes and so on. Use as an anode in cathodic protection has also been proposed and as cathodes in certain processes.
Depending on the use, these dimensionally stable valve metal electrodes have been proposed with various configurations such as rods, tubes, plates and complex structures such as an array of rods or blades mounted on a supporting current conducting assembly as well as a mesh of expanded valve metal typically having diamond shaped voids mounted on a supporting current conducting assembly which provides the necessary rigidity.
Electrodes in the form of platinized valve metal wire are known for cathodic protection, but in practically every other application rigidity and dimensional stability of the electrode are critical factors for successful operation. For example, many electrolytic cells are operated with an inter-electrode gap of only a few millimeters and the flatness and rigidity of the operative electrode face are extremely important.
For most applications, the dimensionally stable electrodes operate at relatively high current densities, typically 3-5 KA/m.sup.2 for membrane cells, 1-3 KA/m.sup.2 for diaphragm ceris and 6-10 KA/m.sup.2 for mercury cells. These high current densities, combined with the requirements of planarity/rigidity, necessitate valve metal structures of substantial current carrying capacity and strength.
Typical known valve metal electrodes of the type with expanded titanium mesh as operative face use a mesh having an expansion factor of 1.5 to 4 times providing a void fraction of about 30 to 70 percent. Such titanium sheets may be slightly flexible during the manufacturing processes but the inherent elasticity of the sheet is restrained, e.g. by welding it to a current conductive structure, typically having one or more braces extending parallel to the SWD dimension of the diamond-shaped openings. Such electrode sheets typically have a current-carrying capacity of 2-10 KA/m.sup.2 of the electrode surface.
Other electrode configurations are known for special purposes, e.g., a rigid cylindrical valve metal sheet mounted in a linear type of anode structure for cathodic protection (see U.S. Pat. No. 4,515,886).
Manufacture of the known electrodes usually involves assembly of the electrode valve metal structure, e.g. by welding, followed by surface treatment such as degreasing/etching/sandblasting and application of the electrocatalytic coating by various methods including chemi-deposition, electroplating and plasma spraying. Chemi-deposition may involve the application of a coating solution to the electrode structure by dipping or spraying, followed by baking usually in an oxidizing atmosphere such as air.